Fabrication of silicon micro mechanical structures

ABSTRACT

A method for protecting a material of a microstructure comprising said material and a noble metal layer against undesired galvanic etching during manufacture comprises forming on the structure a sacrificial metal layer having a lower redox potential than said material, the sacrificial metal layer being electrically connected to said noble metal layer.

FIELD OF THE INVENTION

The present invention relates to silicon micro mechanical structures.More specifically, the invention relates to the fabrication of suchstructures using a wet etch operation. Still more specifically, theinvention deals with the protection of such structures against undesiredgalvanic etching.

BACKGROUND OF THE INVENTION, PRIOR ART

Silicon is used in the semiconductor art to manufacture integratedcircuits (ICs), especially for very large scale (VLSI) devices and thelike. Many steps in the fabrication of such devices include a wet etchoperation using aggressive etching fluids such as hydrofluoric acid(HF).

However, corrosion of silicon may occur when it is dipped in HF. Ingeneral, this effect is small, but it can be enhanced when the siliconis highly doped with elements such as Al, As, C, Ga, P, Sb and the like,as is normally the case with silicon used for the manufacture of VLSImodules. Corrosion of silicon may also be enhanced in the presence ofultraviolet (UV) light or with the anodic polarization of siliconstructures by an external potential.

Examples of silicon etching in HF solutions are the corrosion of highlydoped silicon structures in the presence of UV light as used to detectthe junction depth of a pn diode, silicon electropolishing and poroussilicon fabrication.

Such corrosion may also be enhanced without the use of an externalvoltage or illumination. If, in a device, some silicon part iselectrically connected with noble metals, e.g., gold wires, a galvaniccell is formed when the device is dipped in HF solution with a potentiallarge enough to significantly etch the silicon parts. In thisconfiguration, the noble metal plays the role of the cathode and thesilicon functions as the anode. Reduction of protons to hydrogenmolecules takes place at the cathode and silicon etching will result.This etching of silicon increases with the acidity of the solutionbecause more protons are available in more acidic solutions.

The formation of a galvanic cell in a HF solution is illustrated in FIG.1 using silicon and gold as the anode and cathode, respectively. Thefigure shows the variation of current (log i) with voltage (V). Thecorrosion potential between silicon n+ and gold is shown as the point ofintersection of the two curves representing the Si n+ current (anodeside) and the gold current (cathode side). The redox potential ofsilicon surface depends its react chemical composition. Different dopanttypes and concentrations (n, n⁺, p, p⁺) oxidize with different rates andtherefore give different currents.

In many silicon ElectroMechanical MicroSystems (MEMS) devices, such aschemical sensors, micromechanical devices and integrated opticalelements, a combination of noble metal wiring and silicon is commonlyused. Noble metals are used in MEMS fabrication to produce free standingstructures using an relatively aggressive etching solutions such asPotassium Hydroxide. Standard metals like Al would be etched away bysuch solutions. It is to be noted that wiring must be done before thestructure production because processing is much more difficultthereafter.

Silicon oxide films are extensively used in such devices as electricalisolation layers, passivation, or as a masking film. To pattern thesefilms or to remove them at the end of a process, HF based solutions areefficient and widely used wet etching solutions.

When the noble metal and the silicon parts are electrically connected inthe presence of HF, the silicon may be corroded galvanically. To avoidsuch an undesired effect, the straight forward solution would be toprotect either the noble metal or the silicon or both during the HFetch. However, in many MEMS devices, such protection is not alwayspossible. Examples of such MEMS devices include devices having freestanding structures which limit the deposition and the patterning ofprotective films.

Another known technique is to inhibit any silicon etching with acathodic polarization of the silicon by applying a voltage between thesilicon structure and the metal part. However, in order to apply such atechnique, all the metal and silicon structures must be connectedtogether to two contact zones in order to apply a potential. This may bea problem for devices with many isolated structures, because this wouldrequire very complicated and space consuming additional wiring. As isclear to the skilled worker, this would also incur an additional etchstep in an already complex process to remove the extra wiring after thedevice is completed.

It would be desirable to provide a method for protecting silicon micromechanical structures against undesired galvanic etching that overcomesthe above mentioned drawbacks of the state of the art. It would also bedesirable for such a method that can be easily incorporated intoexisting processes.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is now provided a methodfor protecting a material of a microstructure against undesired galvanicetching during manufacture, said structure comprising said material anda noble metal layer, the method comprising forming on the structure asacrificial metal layer having a lower redox potential than saidmaterial, the sacrificial metal layer being electrically connected tosaid noble metal layer.

The present invention advantageously provides a method for protectingsilicon micro mechanical structures against undesired galvanic etchingthat overcomes the above mentioned drawbacks of the state of the art.Such a method can be easily incorporated into existing processes.

The sacrificial metal layer may be formed of aluminum. The material maycomprise silicon. A preferred embodiment of the present inventioncomprises forming the sacrificial metal layer on one side of saidstructure and is subsequently connected to said noble metal layer. Thesacrificial metal layer may be formed on the noble metal layer. Thesacrificial metal layer may be removed with an etch solution afterfabrication of the micro structure. The sacrificial metal layer may beleft on said micro structure after fabrication thereof. The sacrificialmetal layer may be formed in a plurality of pads each connected to adifferent part of the noble metal layer.

Viewing the present invention from another aspect, there is now provideda microstructure comprising: a material; a noble metal layer; and, asacrificial metal layer electrically connected to the noble metal layerand having a lower redox potential than the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be describedshortly, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a graph showing the variation of current with voltage betweensilicon and gold in HF solution;

FIG. 2 is a graph showing the same type of variation as shown in FIG. 1but with the method according to the invention;

FIG. 3 schematically shows a cantilever manufactured according to thestate of the art;

FIG. 4 schematically depicts a cantilever manufactured according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a preferred embodiment of the present invention there is provided anew way to protect silicon against unwanted galvanic etching. Althoughthe invention is applicable to different etching solutions, it willhereinafter be described with respect to an HF etch solution only.

As mentioned previously, the formation of a galvanic cell in a HFsolution is illustrated in FIG. 1 using silicon and gold as the anodeand cathode, respectively. FIG. 1 shows the variation of current (log i)with voltage (V). The corrosion potential between silicon n+ and gold isshown as the point of intersection of the two curves representing the Sin+current (curve 1, anode side) and the gold current (curve 2, cathodeside)

The protection is achieved based on a cathodic protection of the siliconwith a sacrificial layer forming an integrated anodic electrode. By thedeposition of a sacrificial layer having a lower redox potential thansilicon and being electrically connected to the noble metal used, thecorrosion potential is shifted to a voltage where the silicon currentcorrosion is negligible.

This effect is illustrated in FIG. 2, which shows the formation of agalvanic cell in a HF solution using aluminum and gold as the anode andcathode, respectively. In contrast to the galvanic cell of FIG. 1, anadditional aluminum pad has been connected as an integrated anodicelectrode. The corrosion is defined by the intersection of the twocurves representing the aluminum corrosion current (curve 3, anode side)and the gold current (curve 4, cathode side). The corrosion potential isfixed by the Al-Au cell and at this potential the silicon corrosioncurrent is low.

As can be seen from FIG. 2, the silicon corrosion current is drasticallyreduced by shifting the corrosion potential to higher values of log i.It will be appreciated that “Si n+ galvanic current” in FIG. 1 and “Sin+ corrosion current” in FIG. 2 refer to the same. Al is a preferredmaterial for shifting the corrosion potential because it has arelatively low redox potential. However, other materials, such as Cr,Zn, and Mg may also be employed.

In general, to protect a material A which is electrically connected to amaterial B (where the redox potential of A<than B) against corrosionwhile deep in a solution (electrolyte), a material C having a lowerredox potential than A should be attached to B.

The materials with the lower and higher redox potential define thecorrosion potential and from this potential the corrosion current of allother materials connected to them. This is why the silicon corrosioncurrent reduced when the respective corrosion potential is shifted tohigher values of log i.

This cathodic protection by applying a sacrificial layer of a metalacting as an integrated anodic electrode has the advantage that there isno need to embed or otherwise encapsulate the structure to protect itagainst HF solution attack and that no additional wiring is needed.

The only thing that needs to be done is adding a pad layer or film of asacrificial material to the structure. The sacrificial material can beeither placed on a side of the structure to be protected and thenconnected to the noble metal, or it can be applied on top of thestructure, thus saving space. The sacrificial pad should have an largeenough area in contact with the etching solution, in order that itsefficiency is not limited by the maximum current density at the etchingsolution/sacrificial pad interface.

Once the etch is done, there remains a part of the sacrificial materiallayer which can be etched away using an appropriate etch solution or canbe left as it is. However, in the last case, the remaining sacrificiallayer, should not shortcut functional electrical connection. For thatpurpose, the sacrificial film should be structured so as to avoidshortcut. For example, the sacrificial layer can be deposited just afterthe deposition of the noble metal layer and structured at the same time.

FIGS. 3 and 4 show the application of the present invention to themanufacture of a cantilever/tip structure 6 for use in magnetic storagetechnology. Here, gold wiring 8 is connected to the highly doped siliconcantilever 10, and, at the end of the manufacturing process, a finalHF-based etch is needed to remove all the silicon oxide protectionpresent, in particular the highly doped part. In FIG. 3, no additionalsacrificial aluminum pad has been added and it can be seen that afterthe HF etch, the silicon cantilever 6 is corroded, whereas FIG. 4 showsthat, when using an additional aluminum layer 12 connected to the goldwiring 8, no visible corrosion occurs.

In an example of the present invention herein before described, asacrificial aluminum layer is applied to act as an integrated anodicelectrode. However, it will be appreciated that the present inventioncan be used universally as a complementary tool in integrated systemfabrication as a protection against undesired galvanic etching.Different types of galvanic cells may be formed with different materialand etch solutions. The present invention is especially although by nomeans exclusively attractive for use in MEMS applications because itovercomes the difficulties associated with the conventional protectiontechniques as explained above. The present invention is also applicableto CMOS applications where industry trends towards copper wiring (whichhas a high oxidoreduction potential) and silicon on insulator wafer(where basically the transistors are completely isolated) incur galvanicwet etching problems.

The present invention is also desirable where MEMS devices areintegrated with microelectronic circuits, such as in emerging RFmechanical filter designs.

The present invention is attractive for porous silicon applicationsbecause it adds more flexibility to the fabrication of porous and nonporous zones independent of the silicon doping type and concentration.

1. A method for protecting a material of a structure against undesiredgalvanic etching during manufacture, said structure comprising saidmaterial and a noble metal layer, the method comprising forming on thestructure a sacrificial metal layer having a lower redox potential thansaid material, the sacrificial metal layer being electrically connectedto said noble metal layer.
 2. A method according to claim 1, wherein thesacrificial metal layer is formed of aluminum.
 3. A method according toclaim 2, comprising forming said sacrificial metal layer on one side ofsaid structure and connecting said sacrificial metal layer to said noblemetal layer.
 4. A method according to claim 2, comprising forming saidsacrificial metal layer on said noble metal layer.
 5. A method accordingto claim 1, wherein said material comprises silicon.
 6. A methodaccording to claim 5, comprising forming said sacrificial metal layer onone side of said structure and connecting said sacrificial metal layerto said noble metal layer.
 7. A method according to claim 5, comprisingforming said sacrificial metal layer on said noble metal layer.
 8. Amethod according to claim 1, comprising forming the sacrificial metallayer on one side of said structure and is subsequently connected tosaid noble metal layer.
 9. A method according to claim 1, comprisingforming the sacrificial metal layer on the noble metal layer.
 10. Amethod according to claim 1, comprising removing the sacrificial metallayer with an etch solution after fabrication of the structure.
 11. Amethod according to claim 1, comprising leaving the sacrificial metallayer on said structure after fabrication thereof.
 12. A methodaccording to claim 11, comprising forming the sacrificial metal layer ina plurality of pads each connected to a different part of the noblemetal layer.